1. Field of the Invention
The present invention relates to a roll for the smoothing of paper webs, having a hard roll core constructed of metal and having an elastic covering layer provided on the outside of the roll. The covering layer is an elastic matrix material, which is regionally compressible in operation with the roll rotating, for example, by a counter-roll which can be pressed on the roll.
2. Discussion of Background Information
Elastic rolls of this kind are used, for example, in the satining of paper webs. In such a case, one elastic roll forms, together with a hard roll, a press gap through which the paper web to be treated is guided. While the hard roll has a very smooth surface constructed of, for example, steel or chilled cast iron and is responsible for the smoothing of the side of the paper web facing it, the elastic roll acting on the opposite side of the paper web effects homogenizing and compacting of the paper web in the nip. The size of the roll can range from lengths of about 3 m to 12 m and can have a diameter ranging from about 450 mm to 1500 mm. The roll can withstand line forces of up to about 600 N/mm and compressive stresses of up to about 130 N/mm2.
As the trend in paper manufacturing is progressing towards performing satining in an online operation (i.e. towards guiding the paper web exiting the paper machine or coating machine directly through the paper smoothing apparatus/calender), higher demands than previously are made on the rolls of the smoothing apparatus, particularly with regard to temperature resistance. As a result of the high transportation speeds of the paper web required in online operation and the high rotation speeds of the calender rolls associated with this operation, its nip frequency, that is the frequency with which the covering is compressed and relieved of its load again, is increased. This in turn, leads to increased roll temperatures. These high temperatures arising during online operation lead to problems which can eventually lead to the destruction of the plastic coatings in known elastic rolls. On the one hand, with known plastic coatings, maximum temperature differences of around 20° C. are permissible over the width of the roll and, on the other hand, the plastics conventionally used for the coating have a substantially higher coefficient of thermal expansion than the conventionally used steel rolls or chilled cast-iron rolls so that due to an increase in temperature, high axial stresses occur between the steel roll or the chilled cast-iron roll and the plastic coating associated with it.
So-called hot spots, at which a peeling or even a breaking open of the plastic layer occurs, arise due to these high stresses in conjunction with hot regions occurring particularly in spot form. These hot spots occur, in particular, when in addition to the mechanical stresses and the relatively high temperature, crystallization spots exist in the form of, for example, defective adhesive bonds, deposits or above-average recesses in the elastic coating (i.e. due to creases or foreign bodies on the paper web). In these cases, the temperature of the crystallization spots can increase from the normal range 80° C. to 90° C. to more than 150° C., during which the aforementioned destruction of the plastic layer occurs.